The difference being that we select the car that best meets our requirements (including the driver-centric parameters) and then simply start using it (including enjoying driving it). We might add an accessory or two as we go, but we don’t keep thinking about the mods everyday. I doubt we plan our trips with the primary purpose of checking out the car’s performance. While that might happen immediately after we purchase the car, our trips are mostly decided by our work, shopping and vacation choices.
I think the difference is that the car, though it’s a complex system, is designed and put together by the manufacturer. Whereas the hifi system is configured and put together by us. There are far too many components to decide between, and their interplay so unpredictable! Add the room effect and the hundreds of ways in which things can change (cables crossing after home cleaning resulting in distortion for example), that we just never stop ‘balancing/optimising’ it. Imagine if we had the freedom to buy the chassis, gear assembly, braking system et al from manufacturers and put together our own car, we’d then be spending all the time making it work perfectly and probably never get onto the road, except for test drives!